Condenser microphones include a gooseneck microphone for conferences and a tie pin microphone attached to clothes or the like. In these microphones, a microphone capsule 10 and an output module section 20 are separated from each other and connected via a dedicated microphone cord 30 as shown in FIG. 2.
The microphone capsule 10 comprises a capsule case 11 made of, for example, aluminum. A condenser microphone unit 12 including a diaphragm and a fixed pole (not shown) and an impedance converter 13 including an FET (field-effect transistor) are housed in the capsule case 11 acting as a shield case.
The output module section 20 comprises a cylindrical shield case 21 made of a conductive material (e.g., a brass alloy). A circuit board 22 and an output connector 23 are housed in the shield case 21. A voice output component (not shown) including a transformer, a lowcut filter circuit, and an amplifier circuit is mounted on the circuit board 22. In some cases, the output module section 20 is referred to as a power module section.
Generally, a 3-pin output connector defined by EIAJ RC5236 “a latch-lock round connector for sound” is used as the output connector 23 in the condenser microphone. To be specific, the output connector 23 comprises a first pin for grounding (shielding), a second pin used as the hot side of a signal, and a third pin used as the cold side of a signal. The output connector 23 is connected to a phantom power source (not shown) via a balanced shield cable 40. Reference numerals 1, 2, and 3 of FIG. 2 denote the first pin, the second pin, and the third pin, respectively.
The microphone cord 30 is a twin-core shield covered wire which includes a power wire 31 for supplying power to the microphone capsule 10, a signal line 32 for transmitting a voice signal outputted from the impedance converter 13 to the voice output circuit of the circuit board 22, and a shield covered wire 33 for electrostatically shielding the power wire 31 and the signal line 32 and grounding the power wire 31 and the signal line 32.
The shield covered wire 33 of the microphone cord 30 is connected to the capsule case 11 on the side of the microphone capsule 10 and is connected to a ground circuit (not shown) of the shield case 21 and the circuit board 22 on the signal input side of the output module section 20. The first grounding pin of the output connector 23 is connected, on the signal output side of the output module section 20, to the ground circuit of the shield case 21 and the circuit board 22 in a manner similar to the shield covered wire 33.
Incidentally, when strong electromagnetic waves are applied to the microphone cord 30 and the balanced shield cable 40 on the side of the phantom power source, high-frequency current caused by the electromagnetic waves may enter the shield case 21, a loop current path may be formed by the high-frequency current via a stray capacitance C between the shield case 21 and the circuit board 22, and the loop current path may cause noise.
Cellular phones have rapidly become widespread in recent years. When cellular phones are used near a microphone, extremely strong electromagnetic waves are received (for example, in a range of about several cm to several tens cm, an electric field is several tens of thousands times as strong as an electric field generated by commercial radio waves). Thus, the provision of solutions to cellular phones is an urgent necessity in the field of microphones.
As a solution, Document 1 proposes a method of connecting the ground of an electronic circuit to a microphone case via a wire and directly connecting a first grounding pin to the microphone case. The electronic circuit is housed in the microphone case (shield case) and the first grounding pin is included in an output connector. When the technique of Non-patent document 1 is applied to the conventional example of FIG. 2, a circuit configuration of FIG. 3 is obtained.
[Patent Document 1] “Radio Frequency Susceptibility of Capacitor Microphones,” cowritten by Jim Brown and David Josephson, Audio Engineering Society Convention Paper 5720 (page 12, FIG. 8).
According to the method of Document 1, no loop current path is formed by a stray capacitance C between an electronic circuit (circuit board 22) and a microphone case (shield case 21) and no wire is connected from the first grounding pin to the ground (grounding circuit) of the electronic circuit, that is, nothing acts as an antenna. Thus, it is possible to effectively prevent the entry of electromagnetic waves from the balanced shield cable 40 on the side of the phantom power source.
However, in the case of the method of Document 1, the first grounding pin is directly connected to the microphone case, and thus current passes through the microphone case when the phantom power source is used. Therefore, when the first grounding pin is detached from the microphone case for any reason, the microphone case has a voltage of 30 V or higher in the case of a 48-V phantom power source, and thus a person may receive an electric shock with a touch of a hand on the microphone case.
In addition, in the condenser microphone of FIG. 2 where the microphone capsule 10 and the output module section 20 are connected to each other via the microphone cord 30, even when the technique of Non-patent document 1 is applied, it is not possible to prevent electromagnetic waves entering from the microphone cord 30 to the output module section 20 as shown in FIG. 3.